An abandoned research project on invisibility returns to haunt Eureka. Carter and Zoe become tangled in a web of strange occurrences that lead to Carter's disappearance, and the possibility that he might never reappear.

Callie, Eureka's friendly dry-cleaner, reminds Jack that today is his one-year anniversary in town. Jack is too busy to celebrate. Moon rocks at Global Dynamics are sprouting radioactive crystals for unknown reasons; a substance called polyethylene glycol is missing from the town's pharmacy; and Zoe has a minor car accident during her first solo driving excursion. (Her far-fetched excuse is that she hit something invisible.)

Then an explosion rocks the dry-cleaner's. Jack races back inside and rescues Callie from a second blast. Someone has apparently tampered with her unconventional dry-cleaning formula, causing the explosions. Similarly, Henry and Stark discover that the moon rocks are growing crystals because someone has extracted their natural aluminum-26, a radioactive isotope. There are 18 pockets of the deadly substance located throughout Global Dynamics.

Reviewing security camera footage from the pharmacy theft, Lupo spots a box's lid opening all by itself. She and Jack realize that their thief must be invisible. Recalling Zoe's claim about her accident, Jack discovers that his daughter's damaged car has become invisible, too, when another vehicle collides with it trying to park. He gains painful proof of this when he cuts his finger on its unseen cracked windshield.

Finally, all the strange events around town start making sense. A team at Global Dynamics once tried to invent an invisibility formula using aluminum-26, but the government banned their work for safety reasons. Callie was on that team; in fact, her dry-cleaning formula is a legal adaptation of her research. Tellingly, her former teammate Frank Phillips — a reclusive C.I.A. operative and G.D. employee who was obsessed with invisibility — has not been seen (literally) in two weeks. Apparently, he has continued to pursue his now illegal quest by stealing the chemicals he needs.

The pockets of aluminum-26 within G.D. turn out to be emanating from Phillips's escaped test subjects: 18 invisible, irradiated mice — all near death. The animals prove that although Phillips has discovered an invisibility formula, he can neither make things visible again nor heal the lethal radiation-poisoning caused by the process.

That's too bad, because Jack's arm has just vanished. The infectious invisibility formula on Zoe's car was transferred to him when he cut his finger. As he fades from view, Jack races to track down Phillips. He and Fargo find an invisible lab close to the scene of Zoe's accident. Nearby, they find Phillips himself, killed by his own formula. Callie, Henry, Stark and Allison must now discover how to cure Jack without the inventor's help. If they fail, their sheriff will die of radiation poisoning — and on his first anniversary as a Eurekan.